In recent years, the trend of saving labor has been advancing on motor vehicles and the like, and for example, systems have been under development in which gearshift in a transmission or application of a parking brake of a motor vehicle is performed not manually but by virtue of power of an electric motor. There may be a case where a ball screw mechanism is used on an electric actuator which is used for the aforesaid application in order to convert rotational motion transmitted from the electric motor to axial motion with high efficiency.
However, although a thread groove is normally formed on a screw shaft and nut of a ball screw mechanism through machining, there is caused a problem that some labor hours are necessary for machining with good accuracy. To cope with this, there is known a technique in which for example, a nut is molded including a thread groove.
In Patent Document No. 1, a technique is proposed in which divided nut parts, each having a shape which results when a nut is divided into two or more parts, are each injection molded, and the divided nut parts so molded are then affixed together so as to form a single nut.
In addition, in Patent Document No. 2, there is proposed a technique in which a ball groove portion member of a nut is formed as being divided in an axial direction at a center of a groove thereof.
Furthermore, in Patent Document No. 3, there is proposed a working process in which injection molding is performed using a core mold which is formed like a shaft having an elongated spiral projection.
Patent Document No. 1: Japanese Utility Model Unexamined Publication JP-UM-A-64-11466
Patent Document No. 2: U.S. Pat. No. 3,009,367
Patent Document No. 3: Japanese Patent Examined Publication JP-B-3088812
Patent Document No. 4: Japanese Patent Examined Publication JP-B-2760455